Some UI elements in the pack are made for 9-slice scaling. However the PNG images made in this pack (like the Big buttons in this pack) are separated into 9 parts, however Godot don't use it like that. In Godot you need a full normal images not sliced and you create the margins yourself in the inspector. So if you want to use those banners/buttons you have to stitch them back up into a singular piece (You can use Aseprite, I use Libresprite which is 100% more free than Aseprite and has all basic features, its good for someone who is still learning) , then you can add them into Godot. For example: I have used a normal button node (not textured button) and changed the 'Style' that is under 'Theme Override' in the inspector to add the Big button PNG (The one stitched back together), then in 'Styles' option I changed the 'texture margins' to a certain point (trial and error) , and made 'axis stretch' to tile (this is important if you want to have an expanded button or banner without any stretching effects).
Look up 9-slice scaling if you don't know what it is, and how to use 9-slice in Godot.
and let me know if it works for you.